1. Field of the Invention
The present invention relates to load control circuits for controlling the amount of power delivered to an electrical load from an alternating-current (AC) power source. In particular, the present invention relates to a gate current sensing circuit for determining whether a bidirectional semiconductor switch is conducting a load current to an electrical load.
2. Description of the Related Art
A conventional wall-mounted load control device is mounted to a standard electrical wallbox and is coupled between a source of alternating-current (AC) power (typically 50 or 60 Hz line voltage AC mains) and an electrical load. Standard load control devices, such as dimmers and motor speed controls, use a bidirectional semiconductor switch, such as a triac, or one or more field effect transistors (FETs), to control the current delivered to the load, and thus, the intensity of the lighting load or the speed of the motor. Typical load control devices have a line terminal (or hot terminal) coupled to the AC power source and a load terminal (e.g., a dimmed hot or a switched hot terminal) coupled to the electrical load, such that the semiconductor switch is coupled in series between the source and the electrical load. Using a phase-control dimming technique, the dimmer renders the semiconductor switch conductive for a portion of each line half-cycle to provide power to the lighting load, and renders the semiconductor switch non-conductive for the other portion of the line half-cycle to disconnect power from the load.
Some load control devices, such as “smart” dimmers, include a microprocessor or other processing means for providing an advanced set of control features and feedback options to the end user. The advanced features of a smart dimmer may include, for example, a protected or locked lighting preset, fading, and double-tap to full intensity. To power the microprocessor, smart dimmers include power supplies, which draw a small amount of current through the lighting load each half-cycle when the semiconductor switch is non-conducting. The power supply typically uses this small amount of current to charge a storage capacitor and develop a direct-current (DC) voltage to power the microprocessor. An example of a smart dimmer is disclosed in commonly assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE, which is herein incorporated by reference in its entirety.
Often, it is desirable for the microprocessor of the load control device to determine a state of the electrical load (e.g., the load is on or off). Typical prior art load control devices have detected an electrical characteristic (i.e., a voltage) at the load terminal to thus determine the state of the load. However, the voltage developed at the load terminal is dependent upon the characteristics of the electrical load. Therefore, the voltage developed at the load terminal may be different for different load types. For example, incandescent light bulbs having different wattages also have different impedances. Further, some lighting load types, such as electronic low-voltage (ELV) lighting and electronic ballasts, are characterized by large capacitances, which impact the voltage developed at the load terminal of the load control device.
Thus, there is a need for a load control device having a sensing circuit that is operable to detect an electrical characteristic at a load terminal, but is less dependent upon the voltage at the load terminal and the type of electrical load that the load control device is controlling.